How to find and change material for certain faces?
Materials are assigned to faces.  

Enter edit mode;
change your selection mode to face mode; 
select the face of interest and its material should become highlighted in the material list.

To go the other way (figure out which faces are used by a material);
change your selection mode to face mode; 
deselect all faces; 
choose the material of interest; 
use the Select button beneath the material list (which only appears in edit mode) to select those faces.